Snapshots of Proton Conduction Process in Water

Scientists Capture Snapshots of the Proton Conduction Process in Water

The motion of protons (positively charged H atoms) in water is associated with water’s conduction of electricity and is involved in many important processes including vision, signaling in biological systems, photosynthesis and, the operation of fuel cells. Both artificial photosynthetic systems and fuel cells are of growing interest for clean energy technologies. However, the details of how protons move in water have remained elusive, and an enhanced understanding of the nature of this process is needed to improve the technologies that depend on proton transfer.

An international team of scientists, including a University of Pittsburgh professor and graduate student, has used spectroscopic methods to obtain snapshots of the process by which a proton is relayed from one water molecule to the next. The research is published in a paper in the December 2, 2016 issue of the journal Science.

Ken Jordan, PhD, and Tuguldur Odbadrakh

These measurements represent a major benchmark in our knowledge of how water conducts a positive electrical charge,” said co-author Kenneth Jordan. Jordan is the Richard King Mellon Professor and Distinguished Professor of Computational Chemistry in the Department of Chemistry within Pitt’s Kenneth P. Dietrich School of Arts and Sciences and co-director of Pitt’s Center for Simulation and Modeling. Tuguldur Odbadrakh (Togo) is a graduate student in the Jordan group who contributed to the study. Togo was also a recipient of the PQI Graduate Student Research fellowship in 2014.

This breakthrough involved collaboration of experimental groups led by Mark Johnson of Yale University and Knut Asmis of the University of Leipzig and theory groups led by Jordan and Anne McCoy of the University of Washington. The Johnson, Jordan, and McCoy groups have collaborated for over a decade on the problem of the nature of excess protons in water.

Although the Grotthuss mechanism for proton conduction in water was introduced over two centuries ago, the details have been murky, largely due to the continual and rapid fluctuations of the positions of the water molecules in the vicinity of the excess proton. A key tool in characterizing water networks is infrared spectroscopy, which determines the energies at which the atoms in molecules vibrate. These energies depend sensitively on the relative positions of the molecules. “However, the rapid fluctuations in the positions of the water molecules cause a blurring of the vibrational spectra, thereby limiting the usefulness of this approach, as usually carried out, to elucidate the proton motion,” said Jordan.

The experimental team members solved this problem by obtaining the vibrational spectra of very cold clusters, which contain a small number of water molecules, and by switching from “regular” water to “heavy water”—water incorporating the deuterium isotope of hydrogen. Under these conditions, the vibrational signatures became dramatically sharper, making it possible to obtain a series of snapshots along the proton transfer pathway. Insights into the physical mechanism determining the pathway were provided by calculations carried out in the Jordan and McCoy groups. The calculations revealed that the electric fields imposed on the excess proton by nearby molecules play a major role in establishing the proton transfer pathway.  The understanding of the factors influencing proton transfer gained in this study can be used in designing more efficient materials and devices for energy applications.

Other authors of “Spectroscopic snapshots of the proton-transfer mechanism in water” include Tuguldur Odbadrakh (Pitt), Conrad Wolke (Yale University), Joseph Fournier (University of Chicago), Laura Dzugan (The Ohio State University), and Matias Fagiani and Harald Knorke (University of Leipzig).

Financial support for the research came from the U.S. Department of Energy, the National Science Foundation, and the Collaborative Research Center of the German Research Foundation DFG. Computational resources were provided by the Ohio Supercomputing Center and Pitt’s Center for Simulation and Modeling.